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Questions about rainbows

  1. Apr 26, 2015 #1
    I'm interested in rainbows.

    I'm talking about good old fashioned arc/halo rainbows like those you see in the sky.
    1. Sometimes you see rainbows from lawn sprinklers. How small would the smallest rainbow arc conceivably possible to view be?
    2. Could I make a rainbow at night using a sprinkler with a torch?
    3. If so, how bright would my torch light need to be?
    4. Will even minute drops of water i.e. mist produce rainbows? Mistbows?
    5. Do the water droplets have to be travelling in a consistent direction at a consistent speed?
    6. What factors make the colours in rainbows more saturated and the bands more defined?
    7. Can you be above a rainbow and look down on it without it losing its form?
    I hope you can help!


  2. jcsd
  3. Apr 26, 2015 #2
    A lot of questions there.
    I can tell you the answer to the last one from personal experience.
    If you are in a plane, it is possible to see a whole rainbow, the full circle, it's characteristics are the same as a half rainbow which can be seen from the ground.
  4. Apr 27, 2015 #3


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    1. I'd guess somewhere close to the size of a water droplet, but that's mostly just a guess.
    2. I believe so. I think the key would be to make sure you're viewing from the right angle, which I think is 42 degrees from the light source.
    3. Offhand I'd say the brighter the better.
    4. They will indeed: http://en.wikipedia.org/wiki/Fog_bow
    5. I don't think so, but you may get interesting effects if they are moving around in certain manners: http://en.wikipedia.org/wiki/Rainbow#Twinned_rainbow
    6. Not sure.
    7. No, because a rainbow is not a physical object, so you cannot be 'above' it. In order to see the rainbow you have to be looking towards water droplets at a specific angle from the light source. Any other angle and you won't see a rainbow.
  5. Apr 27, 2015 #4


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    6. The saturation of the bands will depend a lot on the background. You see the sum of light going from a particular direction so a dark cloud behind where the bow appears to come from will make it more 'vivid'. I guess you could arrange an artificial form of rainbow with a black background (possibly a cave mouth). The problem is that the bow is formed of sunlight coming over your shoulder and that light will illuminate the background as well.
    My opinion is that we are so stunned by the effect of a rainbow that we forget to notice just how desaturated the colours are. Do the old eye-dropper test in Photoshop (other packages are available) and look at the RGB values in your very best photo of a rainbow and notice that there is very little difference between the three. We look with our hearts and not ur heads, I think.

    The colours you get from the interference off oil films on water can be much more saturated. It's not the same phenomenon, of course and the interference effect will give you 'notches' in the spectrum, rather than peaks. So, more light in the first place and the background can be very dark.
  6. Apr 27, 2015 #5


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    The colours of a rainbow are formed by the refraction of light which can be done by a very small prism. You could make a straight one too small to see. However if you want to see a proper arc shaped rainbow then you need to look at how the arc shape is formed..


    The size depends on the refractive index of the droplets not their size.

    I don't see why not.

    How bright can your eyes see?
    I believe it makes a difference how many water drops there are per cubic meter.

    Not normally. What matters is the relative position of you and the light source. Due to the way a rainbow is created it forms on the opposite side of you to the sun. eg if the sun is to the west of you the rainbow will be to the east. Change the position of you or the sun and the position of the rainbow will move. You can sometimes see a rainbow below you if you are in an aeroplane but it's not the same one that someone on the ground might be seeing. Everyone sees a rainbow in a slightly different place because their and their eyes are in a different place.
  7. Apr 27, 2015 #6
    As a slightly whimsical answer, there's a reason why we still don't have that pot of gold; it's just impossible to be at the end (or on top) of a rainbow :)
  8. Apr 27, 2015 #7


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    It actually isn't.

    I have stood in a volume of mist and the rainbow was effectively at zero distance.
    I have also flown in a plane and looked down upon a rainbow.
  9. Apr 27, 2015 #8
    Unless I am mistaken, the radius of the rainbow is proportional to the viewer's distance to it, no? Meaning, when you stand right in front of the mist, its extent is 0.

    I think this one just comes down to how one defines "on top" :smile:
  10. Apr 27, 2015 #9


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    Well, it's extent is zero if its distance is zero - i.e. at my eyeball. But how close does one need to be to 'be' somewhere, such as a the end of a rainbow, or to assess whether there is a pot of gold there? Six feet?
  11. Apr 27, 2015 #10


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    The "Engineer's" answer to this would be that, if you started to use a spade and went to reach into the hole you had dug, the 'end' of the rainbow would move away from you. So, close but no cigar (gold). :frown:
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